[RFC] PPC: MPIC: necessary readback after EOI?
b43198 at freescale.com
Tue Jan 6 01:14:37 AEDT 2015
While doing some performance testing of a KVM guest on a PPC platform, I
noticed that there's a read of the CPU_WHOAMI register after each MPIC
EOI . This has been present since the initial implementation of the
MPIC driver . In a KVM virtualized environment, this results in an
Is the read back necessary? Is it used to provide some sort of
synchronization mechanism, making sure that nothing else is executed
until the EOI write is finished? I eliminated the mpic_cpu_read call and
run the kernel on hardware and noticed no anomaly, however I am not sure
of all the implications and race conditions it might lead to.
I was curious why the mpic_cpu_read(MPIC_INFO(CPU_WHOAMI)) was there in
the first place and if it's still needed. If it's still required, I
guess a better approach is to eliminate the call only if the kernel is
running on the KVM guest side, where the MPIC is emulated and no longer
requires a readback.
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